#include <regex>
#include <stdio.h>

int main() {
    std::regex rx("(cat)\\1", std::regex::basic);
    puts("Constructed without throwing: oops!");
    try {
        std::regex_match("(cat)", rx);
    } catch (std::exception& e) {
        puts("regex_match threw instead: oops!");
        puts(e.what());
    }
}

The output is:

Constructed without throwing: oops!
regex_match threw instead: oops!
The expression contained an invalid back reference.

I claim that the constructor should detect and throw on this pattern, rather than throwing at match time. The same issue happens with the "std::regex::grep" dialect, too.